Partly because the AR-15 is lighter than the Mini 14, is easier to reload, and is generally designed to meet the modern needs of armies killin’ humans better. Then there’s the incessant marketing, the huge number of manufacturers at multiple price points (the Mini 14 being a Ruger exclusive), the aftermarket of optics and tacticool accessories, and the general cultural impact. How many Mini 14s have actually been involved in mass shootings and gun-nerd intimidation exercises? It’s almost like the least stable assholes are interested in a “badass” gun.
But okay, fine. There’s a not-insignificant amount of truth to the graphic. By all means, the gun nerds should put it everywhere and inform the previously ignorant public. I don’t think the result will be to convince people the AR-15 is actually useful, just that the Mini-14 is equally unnecessary as a civilian tool or hunting rifle, and they shouldn’t assume a wooden-stock rifle is inherently less dangerous than a plastic one.
And, for the record, I am tediously, annoyingly aware of current second-amendment jurisprudence and the lack of sufficient political will to change the constitution, and while I don’t think the former is well considered, the situation is what it is. It just sucks. It leaves America unique among stable democracies in having gun violence anywhere near the top of the list of causes of death.